This invention relates to electric motor controls, and more particularly to devices and methods for mitigation of torque ripple in an electric motor.
Torque ripple created by permanent magnet synchronous machines (PMSM) is an issue in numerous applications. Traditionally, approaches to mitigate torque ripple have focused upon one of two methods. In the first, mitigation is achieved through machine design (i.e. adjusting magnet/tooth/slot geometry, etc). In the second, harmonics of stator current are controlled to minimize torque ripple. The focus of the research presented herein is on control-based mitigation techniques.
For the purposes of discussion it is convenient to place previous efforts on control-based mitigation in three categories. In [1-17] the parameters of a machine are used to derive stator excitation that minimizes torque ripple (or minimizes subject to constraints such as maximizing efficiency, a fixed dc-link voltage, etc.). More specifically, in [1-6] stator current waveforms are derived using knowledge of the stator back-emf. The ripple from cogging torque is neglected. In [7-13] the ripple from cogging torque is included in the derivation of the respective controls. In [14-17], mechanical parameters of the machine/load (i.e. the moment of inertia and damping coefficient) are used as part of a torque ripple estimator. Detected fluctuations in rotor position are used to estimate torque ripple.
Despite the documented success of the proposed methods, a shortcoming of the schemes proposed in [1-17] is the need for precise knowledge of machine parameters. This limits their applicability to low-volume (custom) applications. To reduce dependence on machine parameters, a second category of mitigation schemes employ parameter estimation techniques. Specifically, in [18-23] measurements of stator current and voltage are used to estimate the back-emf coefficients that are used to establish stator current harmonics. It is noted that the ripple due to cogging torque is not considered in the schemes of [18-23].
Most recently, measurement feedback has been used to provide closed-loop control. Specifically, in [24], a closed-loop torque ripple mitigation scheme is proposed that uses a torque transducer to provide feedback. In [25-26], torque ripple induced vibrations/noise are used for feedback. Specifically, in [25], an accelerometer/microphone is used to provide vibration/noise feedback for mitigating the effects of cogging torque for a PMSM with a sinusoidal back-emf. In [26], a feedback-based method is established to control torque ripple in machines with an arbitrary back-emf and cogging torque. In addition, a lower-cost vibration sensor is used in lieu of an accelerometer.
The techniques of [24-26] represent an advance toward applying control based mitigation methods in mass-produced drives where appreciable variation in machine parameters is expected. As the cost of computing power continues to decrease, such techniques may represent a viable alternative (or a complimentary approach) to machine design-based methods. Despite their promise, a barrier to widespread use of these and nearly all documented control-based torque ripple mitigation schemes has been their reliance on high-precision position encoders. Stated another way, with the exception of [6], all ripple control schemes have been validated using high precision position encoders. In [6], a back-emf detection scheme is proposed to establish rotor position. However, neither the position observer nor the torque ripple mitigation control were demonstrated in hardware. Moreover, the techniques proposed in [6] require precise knowledge of machine parameters.